1. Field of Invention
The present invention relates in general to control devices for controlling internal combustion engines, and more particularly to the control devices of a type that, for achieving desired operation of the engine, controls the movement of intake and/or exhaust valves in accordance with operation condition of the engine. More specifically, the present invention is concerned with improvement of such control devices, by which the lift characteristics (viz., operating angular range, center angle of the range, etc.,) of intake and/or exhaust valves are controlled in accordance with the engine operation condition.
2. Description of Prior Art
Hitherto, various control devices have been proposed and put into practical use in the field of automotive internal combustion engines. Among them, there is a type that controls an operating angular range of an intake valve and a center angle of the operating angular range in accordance with an engine operation condition for obtaining improved fuel consumption and driveability under a low-speed and low-load operation range, and obtaining sufficient engine output under a high-speed and high-load operation range by practically using the advantage of increased mixture charging effect at the intake stroke.
It is now to be noted that the operating angular range defined in the description corresponds substantially to the open period of the intake valve (or exhaust valve) and is represented by an angular range (xc2x0) of the engine crankshaft and the center angle defined in the description corresponds substantially to the center point of the operating angular range or the point assumed when the valve lift shows its maximum degree and is represented by a rotation angle (xc2x0) of the engine crankshaft.
The lecture reference #966 issued from Japanese automotive technology committee in October 1996 shows an engine map (see FIG. 28 of the accompanying drawings) for controlling the operating angular range of the intake valve of a variable valve type internal combustion engine. The engine is equipped with a variable valve mechanism by which the center angle of the operating angular range is continuously changed. In the map, each numeral indicates the crank angle. As is indicated by arrow xe2x80x9cA1xe2x80x9d, under a low-and-medium load operation range, the center angle is advanced with increase of load, while, when the load further increases, the center angle is delayed. While, as is indicated by arrow xe2x80x9cB1xe2x80x9d, under a low-speed operation range, the center angle is advanced with increase of engine speed, while, when the engine speed further increases, the center angle is delayed.
As is understood from the above, under the low-speed and low-and-medium load operation range, the open timing of the intake valve is advanced to increase valve overlapping period thereby to reduce undesired pumping loss. However, during this, the operating angular range is kept unchanged and thus the close timing of the intake valve is inevitably advanced. That is, the close timing of the intake valve is not appropriately controlled. In this case, it is difficult to obtain a desired engine performance, particularly, improved fuel consumption of the engine.
Laid-open Japanese Patent Application 8-177434 shows a valve control device which can vary the valve lift characteristics of the intake and exhaust valves in accordance with engine operation condition. The valve control device disclosed comprises generally a first variable valve actuating mechanism which varies the operating angular range (or valve lift degree) of the intake and exhaust valves in two steps by switching low-speed and high-speed cams and a second variable valve actuating mechanism which varies the center angle of the operating angular range by rotating the cam shaft by a certain degree relative to the crankshaft. In the valve control device of this publication, both the first and second actuating mechanisms are hydraulically actuated by a common oil pump through respective switching valves. Due to usage of the common oil pump, the drive system can be simplified in construction. However, due to inevitability of using such two mechanisms, it is rather difficult to provide precise valve lift characteristics particularly at the time when the control device under the valve switching. Furthermore, due to need of powering the two mechanisms, the oil pump tends to have a larger size.
It is therefore an object of the present invention to provide a valve control device of an internal combustion engine, which is free of the above-mentioned shortcomings.
According to a first aspect of the present invention, there is provided a valve control device of an internal combustion engine having intake and exhaust valves, which comprises a first variable valve actuating mechanism which varies an operating angular range of the intake valve; a second variable valve actuating mechanism which varies a center angle of the operating angular range; and a control unit which controls, through the first and second variable valve actuating mechanisms, the operating angular range and the center angle in accordance with an operation condition of the engine, the control unit being configured to carry out, in a low-output operation range of the engine, advancing the center angle with increase of the engine output while making the variation of the center angle larger than that of the operating angular range; and in a middle-output operation range of the engine, increasing the operating angular range with increase of the engine output while making the variation of the operating angular range larger than that of the center angle.
According to a second aspect of the present invention, there is provided a valve control device of an internal combustion engine having intake and exhaust valves, which comprises first means for varying an operating angular range of the intake valve; second means for varying a center angle of the operating angular range; and control means for controlling, through the first and second means, the operating angular range and the center angle in accordance with an operation condition of the engine, the control means being configured to carry out, in a low-output operation range of the engine, advancing the center angle with increase of the engine output while making the variation of the center angle larger than that of the operating angular range and in a middle-output operation range of the engine, increasing the operating angular range with increase of the engine output while making the variation of the operating angular range larger than that of the center angle.
According to a third aspect of the present invention, there is provided, in a valve control device of an internal combustion engine having intake and exhaust valves, the valve control device including a first variable valve actuating mechanism which varies an operating angular range of the intake valve and a second variable valve actuating mechanism which varies a center angle of the operating angular range, a method for controlling the valve control device in accordance with an operation condition of the engine. The method comprises operating the first and second variable valve actuating mechanisms in such a manner as to, in a low-output operation range of the engine, advance the center angle with increase of the engine output while making the variation of the center angle larger than that of the operating angular range, and operating the first and second variable valve actuating mechanisms in such a manner as to, in a middle-output operation range of the engine, increase the operating angular range with increase of the engine output while making the variation of the operating angular range larger than that of the center angle.